1. Field of the Invention
This invention relates to a semiconductor pressure sensor device having a sensor chip mounted on a resin package and covered with a protective member, for detecting a pressure and producing an electrical signal in accordance with the detected pressure, which is suitably used for detecting an engine intake pressure of an vehicle.
2. Description of the Related Art
A conventional semiconductor pressure sensor device for detecting a negative pressure such as an engine intake pressure of a vehicle or the like is constructed as shown in FIGS. 10A and 10B. The sensor device has a semiconductor sensor chip 102 as a pressure-detecting element. The sensor chip 102 has a diaphragm 102a made of a material (for instance, single crystal silicon) utilizing a piezoresistance effect, and several diffusion resistances (not shown) formed on the diaphragm 102a and connected to form a bridge circuit. Changes in value of resistances of the diffusion resistances caused by the deformation of the diaphragm 102a are taken out of the bridge circuit as electrical signals.
The sensor chip 102 is mounted on a recess portion (sensor mount portion) 103 formed in a resin package 101 through a glass base 105 by adhesive or the like. The sensor chip 102 is electrically connected to conductive members 104, which are insert-molded with the resin package 101, by bonding wires 106. Accordingly, this sensor device can output an electrical signal corresponding to the negative pressure applied thereto.
The sensor chip 102 and the bonding wires 106 are covered with protective members J1, J2 made of insulating materials for protection, electrical insulation, and anticorrosion. In view of a method for forming the protective members in the recess portion 103, there are two kinds of structures, a partially filling structure and a fully filling structure.
FIG. 10A shows the partially filling structure. In this structure, after the sensor chip 102, the bonding wires 106, and the entire surface of the recess portion 103 are covered with a thin film resin J1 made of an organic material, the surface of the diaphragm 102a of the sensor chip 102, connection portions between the chip 102 and the wires 106, and connection portions between the wires 106 and the conducive members 104 are covered with a soft protective resin J2. The protective resin J2 is generally a soft gel-like insulating material such as fluorine-containing gel that can be formed by coating and thermal setting. The thin film resin J1 is, for instance, a palylene (polychloroparaxylylene) film that can be formed by a CVD (Chemical Vapor Deposition) method, which has good adhesion to the protective resin J2.
On the other hand, FIG. 10B shows the fully filling structure in which a protective resin J2 fills the recess portion 103 to cover the sensor chip 102 and the bonding wires 106. According to this fully filling structure, the sensor chip 102 and the bonding wires 106 can be easily covered with soft resin such as gel without forming an organic resin thin film that has a high elastic modulus and necessitates an expensive vacuum deposition apparatus. Therefore, in the fully filling structure, since the protective member produces lower stress than that in the partially filling structure, reliability is improved. In addition, since there is no need to use the vacuum deposition apparatus, manufacturing cost is reduced.
The fully filling structure, however, has a problem that bubbles are liable to be produced in the protective resin J2 due to moisture and substances such as gasoline and exhausted condensed gas contained in environment when the sensor device detects the engine intake pressure of the vehicle. That is, such substances are dissolved into the protective resin J2, and evaporated by heat and pressure to produce bubbles in the resin J2.
The occurrence of bubbles is explained in more detail with reference to FIGS. 11A and 11B. In the fully filling structure, the thickness of the protective resin J2 is thicker than that in the partially filling structure, and, for instance, is more than 1 mm. Because of this, gases produced by substances dissolved in the protective resin J2 are difficult to be released from the protective resin J2. As a result, the substances and moisture caught inside the protective resin J2 are vaporized when temperature rises, and, as shown in FIG. 11A, remain in the protective film J2 as bubbles K1. The bubbles K2 grow when the temperature further rises or pressure is negative.
As shown in FIG. 11B, the grown bubbles K1 can produce cracks K2 that extends from the inside to the surface of the protective resin J2. The cracks K2 may generate leakage current from the sensor chip 102 or the bonding wires 106 (in the figure, from the bonding wires 106). The bubbles K1 existing in the vicinity of the connection portion of one the wires 106 with the sensor chip 102 or with the conductive member 104 (with the conductive member 104 in the figure) may cause breakage of the wire 106.
The present invention has been made in view of the above problems. An object of the present invention is to prevent bubbles from being produced in a protective member covering a sensor chip and an electrically connecting portion of the sensor chip in a semiconductor pressure sensor device.
According to the present invention, a protective member for covering a sensing portion of a sensor chip and a bonding wire has a saturated swelling coefficient of approximately 7 wt % at most when the protective member is immersed into gasoline having a temperature of 20xc2x0 C. The protective member can prevent bubble from being produced therein. No leakage current and no breakage of the wire occur by bubbles.